An electro-magnetic valve may be used as a tank sealing valve in an evaporated fuel processing apparatus. As shown in FIG. 12 of a patent document 1 (i.e., Japanese Patent Laid-Open No. 2013-113401), such an electro-magnetic valve 100A includes a valve chamber 103 to which a first passage 101 that is connected to a canister and a second passage 102 that is connected to a fuel tank are respectively connected. The electro-magnetic valve 100A also includes a valve body 104 housed in the valve chamber 103 which opens and closes the first passage 101, and an electro-magnetic actuator 105 which drives the valve body 104 toward a valve opening direction.
The valve body 104 lifts from and seats on an annular valve seat 106 formed on an edge of an opening of the first passage 101, for communication and dis-communication between the first passage 101 and the second passage 102 via the valve chamber 103, that is, for opening and closing communications therebetween.
The electro-magnetic valve 100A of the patent document 1 has a pressure cancel mechanism. When a high pressure is applied on an opposite side, that is, an away-from-seat side, of the valve body 104 relative to the valve seat during a valve close time, a drive power for driving the valve body 104 must also be driven at a high power, which may deteriorate responsiveness of the driving operation of the valve body 104.
Thus, a pressure cancel mechanism for canceling differential pressure before and behind the valve body 104, that is, a pressure difference between a valve seat side and an away-from-seat side, at the valve close time is provided as a pressure cancel chamber 108 that is formed behind (i.e., on the away-from-seat side of) the valve body 104 into which a pressure of the first passage 101 is introduced.
For example, in the patent document 1, the pressure cancel chamber 108 is air-tightly defined against the valve chamber 103 by a pressure chamber defining member 109 (e.g., a diaphragm in this example) that is connected to a behind side of the valve body 104. In such a structure, the valve body 104 receives a pressure receiving load Fb of the pressure cancel chamber 108, i.e., on the away-from-seat side via the pressure chamber defining member 109, and the valve body 104 also receives a pressure receiving load F1 from the first passage 101 on the valve seat side.
To make the most of the effect of this pressure cancel mechanism, it is desirable to equalize those two pressures, i.e., the pressure receiving load Fb and the pressure receiving load F1. That is, it is desirable that a pressure receiving diameter Dv of the valve body 104 is configured to be equal to an effective pressure receiving diameter Dc of the pressure chamber defining member 109. In such case, the pressure receiving diameter Dv of the valve body 104 is a diameter of a surface that receives pressure from the first passage 101 at the valve close time of the valve body 104. Further, the effective pressure receiving diameter Dc of the pressure chamber defining member 109 is a diameter computed from an area size (i.e., an effective pressure receiving area size) determined based on “a load (i.e., pressure) put on the pressure chamber defining member 109.”
Further referring to the other document, a patent document 2 (i.e., Japanese Patent Laid-Open No. 2005-291241) discloses an electro-magnetic valve 100B which prevents a steep increase of an opening area size at the time of valve opening as shown in FIG. 13 (in FIG. 13, the same numerals are assigned to the same configuration as the electro-magnetic valve 100A of FIG. 12).
In the electro-magnetic valve 100B, the first valve body 104A, which lifts from and seats on a first valve seat 106a formed on an edge of an opening of the first passage 101, has a communication hole 111 bored therein for communication between the first passage 101 and the valve chamber 103, and the second the valve body 104B provided as another valve body is used to opens and closes the communication hole 111. The first valve body 104A is designated as the switch valve body 104A, because it switches an amount of the flow between the first passage 101 and valve chambers 103. The second valve body 104B is designated as the open-close valve body 104B, because it opens and closes the first passage 101.
The open-close valve body 104B is driven by the electro-magnetic actuator 105, and the switch valve body 104A is biased by a spring 112 in a valve lift direction, and is driven by a pressure difference before and behind the switch valve body 104A.
That is, when the open-close valve body 104B lifts from the seat, the first passage 101 and the valve chamber 103 communicate with each other via communication hole 111 of the switch valve body 104A. In other words, a fluid flows to the first passage 101 from the second passage 102. The switch valve body 104A is in a seated state under the pressure of the valve chamber 103 immediately after the lifting of the open-close valve body 104B. Then, in a short time, as the pressure of the valve chamber 103 (i.e., the pressure of the second passage 102) falls, the switch valve body 104 lifts from the seat according to the biasing force of the spring 112.
Then, the pressure cancel mechanism is also provided in the electro-magnetic valve 100B. In this electro-magnetic valve 100B, a bellows is used instead of the diaphragm as the pressure chamber defining member 109. In this case, too, to make the most of the effect of the pressure cancel mechanism, it is desirable that the pressure receiving diameter Dv2 of the open-close valve body 104B is equated to the effective pressure receiving diameter Dc of the pressure chamber defining member 109, just like the above-mentioned example.
In such case, for the two-stage type electro-magnetic valve 100B having the two valve bodies, a mechanism for keeping a valve seated state is required with which the switch valve body 104A is kept seated immediately after the lifting of the open-close valve body 104B.
Such a mechanism of the electro-magnetic valve 100B is provided as a structure in which (i) the pressure receiving diameter Dv1 of the switch valve body 104A is longer than the pressure receiving diameter Dv2 of the open-close valve body 104B, and (ii) the pressure receiving load by the differential pressure between the valve chamber 103 and the first passage 101 starts to act on the switch valve body 104A while the open-close valve body 104B is still in the seated state. According to such mechanism, the switch valve body 104A is kept in the seated state according to the pressure receiving load.
In other words, the electro-magnetic valve 100B has a restriction that the pressure receiving diameter Dv2 of the open-close valve body 104B must be smaller than the pressure receiving diameter Dv1 of the switch valve body 104A.
When, on the other hand, the pressure receiving diameter of Dv1 of the switch valve body 104A is increased too much, a negative pressure in a fuel tank inducing a negative pressure in an inside of the valve chamber 103 causes the lifting of the switch valve body 104A from the seat even when the open-close valve body 104B is not in a driven state, which is a deficiency of the electro-magnetic valve to be serving as a tank sealing valve. That means, there is another restriction that the pressure receiving diameter Dv1 cannot be increased too much.
Therefore, in the conventional electro-magnetic valve 100B, there are a couple of restrictions in terms of the diameter Dv1 and the diameter Dv2, since the pressure receiving diameter of Dv2 of the open-close valve body 104B is determined, not freely, but by the relationship with the pressure receiving diameter of Dv1 of the switch valve body 104A. In other words, the effective pressure receiving diameter Dc has to be set in the middle of the pressure receiving diameter Dv2 and the pressure receiving diameter of Dv1.
In other words, in the conventional electro-magnetic valve 100B, the pressure receiving diameter of Dv2 of the open-close valve body 104B cannot be equated to the effective pressure receiving diameter Dc of the pressure chamber defining member 109, thereby making it impossible to maximize the effect of the pressure cancel mechanism.